Styrene resins are very common among synthetic resins, and various kinds of copolymers for use in improving properties of styrene resins have been proposed and are being actually used extensively in various fields. Such proposals include, for example, the following: In JP-A-61-223701 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") are described a copolymer of a styrene monomer and a monomer having at least two radical-polymerizable carbon-carbon double bonds in the molecule and a synthetic resin lens composed of the copolymer. In JP-A-62-187712 is described a plate of a crosslinked resin comprising styrene as the major monomer component, which plate has a specific thickness and size, is free from a depressed part on the surface, contains no bubbles inside, and has a degree of swelling of 10 or less. A method of producing this resin plate is also described therein. JP-A-56-167706 discloses a method in which a combination of special organic peroxides is used as a catalyst in order to obtain a polymer which has a low residual monomer content and a high molecular weight, is excellent in mechanical strength, especially in weight drop strength, and is suited for injection molding.
However, the resins disclosed in JP-A-61-223701 and JP-A-62-187712 are defective in that since the whole resins have a crosslinked structure so as to retain strength, they show poor heat formability.
Illustratively stated, when resin plates made of a material having poor heat formability are formed into various shapes by pressing the plates with heating against molds corresponding to the desired shapes, shape transfer from the molds cannot be attained satisfactorily and it is difficult to impart the desired shapes. Although the above problem may be suppressed by heightening the forming temperature, this causes decomposition of the resin and is apt to yield colored formed articles.
The method described in JP-A-62-187712 is disadvantageous in that it necessitates a polymerization time as long as about 8 hours and that the residual monomer content is relatively high. In addition, it is necessary that the gasket for use in this method should be subjected to a special treatment in order to avoid the development of surface defects in the plates to be produced. Styrene resins having a high residual monomer content are inferior in mechanical strength, weatherability, and solvent resistance.
The method of JP-A-56-167706 is disadvantageous in that special organic peroxides should be used and the polymer obtained is still insufficient in mechanical strength, flexural strength, etc.